In recent years, an automatic driving technology of a vehicle is actively researched. Such automatic driving technology includes: a technique of recognizing an obstacle using an on-vehicle sensor and applying an automatic brake when a crash is possible and; a technique of detecting a position/speed of surrounding vehicles and autonomously adjusting the vehicle speed to keep the inter-vehicular distance with a preceding vehicle constant and the automatic driving technology is realized in a part of vehicles.
In future, to realize more high levels of automatic traveling, not only the adjustment for keeping the vehicle speed and the inter-vehicular distance on the same lane, but also a travel control when a vehicle changes a lane, joins/divides to/from a lane, and turns right or left is needed. When the change of the lanes and the joining/dividing to/from a lane are performed, especially it is needed to ensure the safety and the smoothness of the traffic stream.
In a conventional travel control when a vehicle changes the lane, joins/divides to/from a lane, and turns right or left, drivers use a visual contact and a blinker to recognize each other and accordingly a timing to change the lanes and join/divide to/from a lane is adjusted. In the automatic driving technology, it is needed to automatically and smoothly perform the control which is the same as the operations of the driver.
However, a real traffic environment is complicated, and it is very difficult to decide the timing to change the lanes and join/divide to/from a lane only with the on-vehicle sensor such as a camera and a radar. This is because the travel control of an own-vehicle is largely varied depending on an unpredictable traveling behavior of vehicles around the own-vehicle.
Then, in addition to the peripheral recognition with the on-vehicle sensor, it is proposed to collect information around the vehicle using the vehicle-to-vehicle communication or the road-to-vehicle communication technique. For example, PTL 1 discloses a drive assist method for performing the vehicle-to-vehicle communication between the joining vehicle which is traveling on the joining road and the vehicle traveling on the main road and determining whether to change the lanes based on the times needed for the joining vehicle to arrive at a junction. (Japanese Unexamined Patent Publication No. 11-328597)